Oropouche (ORO) fever is vector-borne viral disease, transmitted by the biting-midge Culicoides paraensis, which during the past 40 years has emerged as an increasing public health problem in South America. ORO virus is a member of the Simbu serogroup of the genus Bunyavirus, family Bunyaviridae and has a tripartite genome consisting of large (L), medium (M) and small (S) single-stranded, negative-sense RNAs. The first isolate of ORO virus was made in Trinidad in 1955, from the blood of a febrile forest worker. Subsequently, the first documented epidemic of ORO fever took place in Belem, Para state, Brazil in 1961 and there have been more than 30 additional outbreaks of ORO fever in the Amazon Basin of Brazil and Peru, and from the Isthmus of Panama. The number of infected humans per outbreak has varied from a few hundred to approximately 100,000. Thus, ORO is an emerging viral disease with an increasing geographic distribution which has the potential to cause serious disease in humans and to spread in Cu. paraensis beyond central South America as the vector ranges from northern Argentina to Wisconsin in the United States. Seroepidemiologic studies in Brazil indicate that the prevalence of anti-ORO virus antibodies is 0-2 percent in areas where epidemic transmission has not been reported whereas antibody prevalence increases to 17-44 percent following outbreaks. In comparison, recent studies in Peru reported a seroprevalence of 35 percent in the urban population. Preliminary studies suggest, on the basis of sequencing and serology, that at least one strain of Simbu group virus Inini is a reassortant containing the S-RNA of ORO virus, subtypes of ORO virus exist, and Brazilian isolates of ORO virus from 1980 contain a different S-RNA to other isolates of ORO virus examined. This project will utilize both laboratory and field-based studies to investigate the hypothesis that not all cases of febrile illness clinically defined as ORO fever are caused by ORO virus, rather natural ORO-like virus reassortants that exist in the Amazon region of South America are responsible. To investigate this hypothesis, we will examine isolates of ORO and related Simbu serogroup viruses found in tropical South America available in the World Arbovirus Reference Center at the University of Texas Medical Branch at Galveston. In addition, we propose to undertake field studies in Peru and Brazil to obtain isolates of ORO and related Bunyaviruses, and subject them to molecular epidemiological analyses to look at the genetic relatedness of the viruses and the possibility that natural ORO-like virus reassortants are present in the Amazon region today.